1. Field of the Invention
The invention relates generally to the reduction of oxides of nitrogen (NO.sub.x) in the exhaust gases of furnaces and, more particularly, to an apparatus and method for utilizing a portion of the exhaust gases from a glass melting furnace to form a substantially nitrogen free synthetic air mixture to support combustion in the furnace.
2. Description of the Prior Art
Most commercial glass is produced in high temperature air/fuel furnaces where solid raw materials are melted, reacted to form stabilized silicates and degassed of entrained gases to allow downstream forming of a homogeneous product. Energy input to the furnace in the form of natural gas or oil firing and electrical resistance heating (electric boosting) melts the raw materials, provides heat of reaction and raises the molten temperature while decreasing viscosity to allow for proper degassing of the glass. The vast majority of these furnaces use air to support combustion. Different furnace designs have evolved in each segment of the glass industry which are specifically tailored to the particular demands of the end use product. Examples of traditional furnace designs include the regenerative melter, recuperative melter, all-electric melter and direct fired unit melter.
By far, the dominant furnace design for the glass industry is the regenerative melter. A typical regenerative melter includes at least two burners, two regenerators, a flow reversal system and associated controls. Paired sets of burners are located on opposed sides of the furnace or are end port fired where both systems are on the same wall of the furnace. A heat regenerator communicates with each burner. One system utilizes burners and regenerators that are closely coupled by a length of refractory lined duct to suit the space available on site. Other systems utilize separate burners which are located within an exhaust port. When the first burner of a pair fires, using combustion air fed to the base of its regenerator, the second burner of the pair acts as an exhaust port drawing off waste gas or the more conventional design permits the waste gases to enter an exhaust port, thereby heating the regenerator for the second burner or combustion port. When this heated regenerator is sufficiently charged, the reversal system operates to reverse the firing system. The second burner of the pair fires to heat the furnace and the first burner, in time, acts as an exhaust port, or there is an exhaust port where the burner is incorporated as a stand-alone burner. The combustion air is then directed through the hot regenerator of the second burner or port to preheat the combustion air prior to combustion. After a period of time, the flow of exhaust gases and combustion air through the regenerators is again reversed to maintain heating of the combustion air.
These regenerators typically take the form of latticed brick work or "checkers" through which the combustion air passes on its way to the burner or port to preheat the combustion air and through which the exhaust gases from the furnace pass on their way to the stack. The exhaust gases transfer their sensible heat to the regenerator bricks as they pass through. On the reverse cycle, clean combustion air brought in at ambient temperature is passed through the previously heated regenerator of the firing burner and thus picks up sensible heat from the bricks. In this way, the regenerator preheats the air prior to combustion.
In an alternative traditional furnace design, recuperative heat exchangers, rather than regenerators, are used to preheat the combustion air. Recuperative heat exchangers differ from regenerative heat exchangers in that the exhaust gases and combustion air flow through different piping systems and do not mix. The recuperator acts as a simple indirect heat exchanger. Heat from the exhaust gases flowing through one conduit is transferred to combustion air flowing through another conduit.
A problem with these known air/fuel furnaces is that due to the use of atmospheric air as the combustion gas, NO.sub.x compounds are produced during combustion. Government environmental regulations strictly limit the amount of NO.sub.x compounds which may be discharged into the atmosphere, thereby requiring costly clean-up to be done on the furnace exhaust gases prior to discharge into the atmosphere.
As an alternative to conventional air/fuel furnaces, oxy-fuel fired furnaces have been developed. In an oxy-fuel fired furnace, pure oxygen gas instead of air is introduced into the furnace to support combustion. The use of pure oxygen rather than air eliminates the NO.sub.x problem generally associated with air/fuel systems. However, unlike conventional air/fuel furnaces, the oxygen is typically not preheated prior to being mixed with the fuel and no flow reversal systems are used. The exhaust gas is simply directed to an exhaust stack. No regenerators or recuperators are generally associated with oxy-fuel fired furnaces.
Oxy-fuel fired furnaces clearly offer some advantages over typical air/fuel furnaces, such as generally lower NO.sub.x concentrations. However, oxy-fuel furnaces also have some strong disadvantages. Oxy-fuel furnaces, as a general rule, are more expensive to operate since the oxygen must be purchased for use in the furnace. Further, oxy-fuel furnaces are typically not as thermally efficient as regenerative or recuperative furnaces. Additionally, special burner nozzles are required for oxy-fuel fired furnaces since the oxy-fuel system burns at a much higher temperature than conventional regenerative or recuperative furnace systems. Therefore, for many glass manufacturers, the higher costs required to switch from a regenerative or recuperative system to a conventional oxy-fuel system outweigh the benefits derived from the conversion.
In an effort to reduce the emission of NO.sub.x in air/fuel systems, furnace systems have been developed in which a portion of NO.sub.x containing stock gas is recycled, in admixture with cold fresh air, back into the combustion chamber of the furnace. U.S. Pat. No. 4,699,071 is an example of such a system. U.S. Pat. Nos. 3,760,776; 4,926,765; 4,995,807; and 5,040,470 are also examples of combustion systems in which a portion of the flue gas is recirculated to support combustion. However, such recirculation systems suffer from stable combustion problems and also do not totally eliminate the emission of NO.sub.x from the discharged air. Additionally, these systems do not fully utilize the benefits of a heat recovery system to increase the operating efficiency of the systems.
U.S. Pat. No. 3,905,745 to Konda discloses a method and apparatus for preventing the formation of harmful constituents in exhaust gases, including NO.sub.x, through a mixture of recirculated combustion gases and oxygen. However, in the Konda patent, little concern is given to the recovery of waste heat and the additional energy savings that can be realized by such waste heat recovery.
Therefore, it is an object of the invention to provide a method and assembly for cost-effectively retrofitting existing regenerative and recuperative furnace systems to use a "synthetic air" comprising a mixture of exhaust gases and oxygen as a combustion gas source while maintaining the benefits of waste heat recovery. It is also an object of the invention to provide a cassette regenerative oxy-fuel furnace system which utilizes a synthetic air mixture of exhaust gases and oxygen which substantially eliminates the discharge of NO.sub.x into the atmosphere as well as reducing the particulate emissions.